Basic Information

A species could inhale methane, or exhale methane during its respiration. For the former, they would need to have an oxidant in their food supply, however. The idea may suit a world with a lot of carbon and little oxygen, such as a Carbon Planet. A species that consumes methane is known as a methanotroph, and many are bacteria that use oxygen to feed on methane as it seeps out from rocks. This is not suitable for a wider biosphere as there are just too many opportunities for the flammable methane-oxygen mixture to burn away. For the oxidant, there are better options than oxygen.

As an example of the way such an ecosystem might work, the animals may be Sulfur-Breathing Life as well. If they breathed methane, as it happens, they would not need hellish temperatures, whether as heat that melts the raw element or freezing temperatures that make hydrogen sulfide into a liquid, for sulfur to be a viable option. As the plants collect energy from light, they would accumulate yellow sulfur and sulfates as wastes. The animals would eat the plants and respire the sulfur with methane to yield energy. When dead matter decomposes, it releases the methane back into the air as creatures which exhale methane start to digest it.

The creatures that exhale methane are known as methanogens, and they are known to exist as small micro-organisms on Earth, but they have been suggested as a way to explain anomalies in the atmosphere of Mars and Titan. They would breathe hydrogen from the fermentation of organic matter and react it with carbon dioxide or organic compounds to make methane and water. The methane can be recycled back to hydrogen and organic compounds by natural processes as well as living things. This is a little less straightforward than the sulfur cycle, or the oxygen cycle we rely upon, and may be less likely to form.

Due to the fact that energy and carbon may be extracted from atmospheric methane and the sulfur byproducts, food is not necessary as a source of energy, only nutrients, and many nutrients can be extracted or synthesized with enough energy. The ecosystem may be a lot gentler as a result, as the incentives are changed. If you can't digest the energy-rich fat in your prey without sulfates that come from plants anyway, why be a predator? Without any predators, there is no reason to hide away, so fearlessness, ritual behaviors, bright colors, and sexual selection may erupt into a vibrant and diverse ecology, making the methane-sulfur world into a Garden of Eden, as serene as a hotter sulfur world is infernal.

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Game and Story Use

Any planets with a methane-sulfur cycle might be very significant for it.

They might appeal to people who want to harm no living thing, who would use genetic engineering to integrate themselves into the ecology.

There is potential symbolism in the contrast between a hot planet with lakes of red sulfur and an idyllic, cooler world with no predators.

An intelligent species that evolved on a methane-sulfur world might be disturbed by the idea of carnivores and meat, although ritual duels to the death might be quite normal to them.

After millions of years of evolution ruled by sexual selection and ritual behavior, without the pragmatic need to escape predators, you could justify pretty much any weirdness in the wildlife you want.

The metabolism used by methanogens would be very useful in extreme conditions without oxygen, as methane, hydrogen and simple organic compounds are quite common. Even a Gas Giant could bear suitable habitats for them, with luck and the right adaptations.